Protection circuit for switching mode power supply circuit

ABSTRACT

A protection circuit for a switching mode power supply circuit which outputs a constant voltage interrupts the ON-OFF switching operation of the switching transistor when an overvoltage state is detected. This protection circuit includes a switching transistor, a diode, a phototransistor, a thyristor, and a capacitor, in conjunction with a photodiode, a zener diode and a thyristor. If either an overvoltage is detected or an overcurrent is detected due to the load circuit being short circuited, the protection circuit simultaneously prevent the power supply input from the power input line from being unnecessarily lost.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a switching mode power supply circuit (SMPS) using repetitive switching of the states which ON and OFF of a transistor to cause a direct current being applied to a primary coil of transformer to induce a current into a secondary coil, and more particularly, to a protection circuit for a switching mode power supply circuit using the ON and OFF operation of a transistor to allow an interruption when an over voltage is outputted.

2. Discussion of the Prior Art

As shown in FIG. 1, a protection circuit for a switching mode power supply circuit is constructed such that a power input terminal V_(IN) is connected to a collector of a transistor TR₁ switching ON and OFF a current flowing through a primary coil T₁₁ of a transformer T₁ ; an emitter of the transistor TR₁ is connected to an intermediate terminal of a secondary coil T₁₂ of transformer T₁ ; the end terminal of the secondary coil T₁₂ are connected to a base of the transistor TR₁ through a resistor R₁₄, a capacitor C₁ and a feed back circuit 1; secondary coils T₁₃ T₁₄ are, respectively, connected to each input terminal of a rectifier 2 which is composed of a diode D₁, capacitors C₂ C₃ and a coil L₁ and another rectifier 3 which is composed of a diode D₂, capacitors C₄ C₅ and a regulator REG. On the other hand, an output terminal of the rectifier 2 is connected to a zenor diode ZD₁ and a resistor R₂ through a resistor R₁. Resistor R₁ is also connected to an emitter of the transistor TR₂ through the resistor R₃ and to an inversion input terminal (-) of a comparator COMP through a resistor R₄. The output terminal of rectifier 2 is also connected to a resistor R₆ and to a non-inversion input terminal (+) of the comparator COMP through a resistor R₅. The output terminal of comparator COMP is connected to a base of the transistor TR₂ through a resistor R₈. A collector of transistor TR₂ is connected to a base of a transistor TR₃ through a resistor R₁₀. An emitter of the transistor TR₃ is connected to a gate of a thyristor SCR₁ and a capacitor C₆ through a resistor R₁₃. An anode of thyristor SCR₁ is connected the diode D₁, capacitor C₂, and coil L₁.

The operation of the conventional circuit will be explained below.

When a direct current is inputted to a power input terminal V_(IN), the direct current power is applied to a primary coil T₁₁ of transformer T₁, thereby inducing a current in the secondary coils T₁₂ and T₁₃ in accordance with the ON and OFF switching of the transistor TR₁. The power induced in a secondary coil T₁₂ is applied to a base of transistor TR₁ through the feed back circuit 1, thereby causing the ON and OFF switching of the transistor TR₁ to be repeated.

At this moment, the power is induced in another secondary coil T₁₃ and is rectified via the rectifier 2 which outputs a constant voltage V₁. At the same time, the outputted constant voltage V₁ is applied to a zenor diode ZD₁ through a resistor R₁ to becomes another constant voltage. It is then applied to an inversion input terminal (-) of the comparator COMP through a resistor R₄. Further, the power is induced in to another secondary coil T₁₄ is rectified via the rectifier 3 which outputs a constant voltage V₂ the output voltage V₂ is applied to a non-inversion terminal (+) of the comparator COMP through a resistor R₅.

If the voltage which applied to the inversion input terminal (-) of the comparator COMP is lower than the voltage of non-inversion input terminal (+) of the comparator COMP, a high potential signal is outputted from the comparator COMP and the transistor TR₂ is turned OFF, accordingly the transistor TR₃ also turns OFF. In response to this OFF state, the gate of the thyristor SCR₁ is applied with a low potential, and the thyristor SCR₁ is turned OFF, thereby causing the rectifier to execute normal operations.

If the voltage applied to the inversion input terminal (-) of the comparator COMP is higher than the voltage applied to the non-inversion input terminal (+) of the comparator COMP, a low potential signal is outputted from the comparator COMP, and in contrary with above description, the transistor TR₂ turns ON causing the transistor TR₃ to also turn ON. A high potential signal is outputted to its emitter. This high potential signal is applied to the gate of the thyristor SCR₁ thereby causing the thyristor SCR₁ to become conductive and rendering the operation of the rectifier 2 to be terminated.

However, in such a conventional protection circuit for switching mode power circuit, if an overvoltage is outputted from the rectifier 2, the switching transistor TR₁ continuously repeats switching ON and OFF; therefore, the direct current power input to the power input terminal V_(IN) is unnecessarily lost such that the, switching mode power supply circuit cannot be protected.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a protection circuit for a switching mode power supply circuit which protects the switching mode power supply circuit from an overvoltage condition.

The object of the present invention, as described above, is realized in such a manner when an overvoltage is outputted from the switching mode power supply circuit, the first constant voltage is terminated by interrupting the ON.OFF switching operation of the switching transistor.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a circuit diagram of a conventional protection circuit for a switching mode power supply circuit.

FIG. 2 is a circuit diagram of a protection circuit for a switching mode power supply circuit according to an embodiment of the present invention.

FIG. 3 is a circuit diagram of a protection circuit for a switching mode power supply circuit according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 2, a switching mode power supply circuit is constructed such that a power input terminal V_(IN) is connected to a collector of a switching transistor TR₂₁ through a primary coil T₂₁ of a transformer T₂. An emitter of the transistor TR₂₁ is connected to an intermediate terminal of a secondary coil T₂₂ and one end terminal. The other end terminal of the secondary coil T₂₂ are connected to a base of the transistor TR₂₁ through a resistor R₂₁, a capacitor C₂₁, and a feed back circuit 221. The secondary coils T₂₃ and T₂₄ of the transformer T₂ are connected, respectively, to each of the input terminals of rectifiers 22 and 23 so that constant voltages V₂₁ and V₂₂ are outputted.

The output terminal of the rectifier 23 is connected in common to a photodiode PD₂₁, a resistor R₂₃, a capacitor C₂₆, a gate of a thyristor SCR₂₁ through a resistor R₂₂ and a zenor diode ZD₂₁. The anode of the thyristor SCR₂₁ is connected to the output terminal of the rectifier 22 through the resistor R₂₄. The emitter of a phototransistor PTR₂₁ which senses the light of the photodiode PD₂₁ is connected in common to a gate of the thyristor SCR₂₂, a resistor R₂₅ a capacitor C₂₇, a connecting point between the resistors R₂₆ and R₂₇ which are connected in series to the power input terminal V_(IN). The connecting point between resistors R₂₆ and R₂₇ is connected to an anode of the thyristor SCR₂₂ and to a collector of the phototransistor PTR₂₁. A base of the switching transistor TR₂₁ is connected to resistor R₂₇ and capacitor C₂₁.

The operation of the present invention will be explained below.

When direct current power is inputted to the power input terminal V_(IN), the direct current power is applied to a primary coil T₂₁ of the transformer T₂ to induce a current in the secondary coils T₂₂, T₂₃ and T₂₄ in accordance with the ON and OFF switching of the transistor TR₂₁. The power induced in the secondary coil T₂₂ is applied to the base of the transistor TR₂₁ through the feedback circuit 21, thereby causing the ON and OFF switching of the transistor TR₂₁. The power induced in the secondary coils T₂₃ and T₂₄ are rectified via the rectifiers 22 and 23 and to produce the constant voltages V₂₁ and V₂₂.

When a rated voltage is outputted a zenor voltage of the zenor diode ZD₂₁ is established higher than the rated voltage to cause the zenor diode ZD₂₁ to turn OFF at this moment. Accordingly at this moment, the photodiode PD₂₁ is not lit, and the thyristor SCR₂₁ is OFF turned. Therefore, a rated voltage V₂₁ is normally outputted from the rectifier 22, and the phototransistor PTR₂₁ is turned OFF state in accordance with the lit off of the photodiode PD₂₁ ; therefore, the switching transistor TR₂₁ becomes to repeats the switching ON and OFF operations. If an overvoltage is outputted from the rectifier 23 and applied to the zenor diode ZD₂₁, the zenor diode ZD₂₁ becomes conductive. Therefore, a rated voltage V₂₂ outputted from the rectifier 23 is applied to the photodiode PD₂₁ and to a gate of the thyristor SCR₂₁ through the resistor R₂₂ and zenor diode ZD₂₁, thereby turning the photodiode PD₂₁ ON to produce light and causing the thyristor SCR₂₁ to become conductive.

Thus, when the thyristor SCR₂₁ becomes conductive, the constant voltage V₂₁ outputted from the rectifier 22 flows to earth through the resistor R₂₄ and the thyristor SCR₂₁.

Further, since the phototransistor PTR₂₁ is ON in response to the lighting of the photodiode PD₂₁, the power of the power input terminal V_(IN) is applied to a gate of the thyristor SCR₂₂ through the resistor R₂₆ and phototransistor PTR₂₁. In accordance with this, the power applied to a base of the switching transistor TR₂₁ flows to the thyristor SCR₂₂ through a diode D₂₃ ; therefore, the switching transistor TR₂₁ is not operated in a switching operation and remains OFF.

FIG. 3 is a circuit diagram showing the another embodiment of a protection circuit for switching power circuit according to the present invention. The protections circuit is constructed such that a connecting point between the resistor R₄₆ and a collector of the transistor TR₃₄ which outputs a second constant voltage V₃₂ is connected to a base of a transistor TR₃₃ through a resistor R₄₇. A collector of the transistor TR₃₃ is connected to the photodiode PD₃₂ through a resistor R₄₈. An emitter of the phototransistor PTR₃₂ which senses the light of the photodiode PD₃₂ is connected in common to a gate of the thyristor SCR₃₁, resistor R₃₆ and capacitor C₃₁. A connecting point of the resistors R₃₂ and R₃₃ which are connected in series to the power input terminal V_(IN) is connected in common to an anode of the thyristor SCR₃₁ and to a collector of the phototransistor PTR₃₂. A base of the switching transistor TR₃₁ is connected in common to an anode of the thyristor SCR₃₁ and to a collector of the phototransistor PTR₃₂ at through a diode D₃₂.

The elements connected between a terminal end of secondary coil T₃₂ of the transformer T₃ and a base of the switching transistor TR₃₁ are formed with the usual feedback circuit. The resistors R₃₂, R₃₅, R₃₈ and R₄₁, capacitor C₃₆, transistor TR₃₅ and phototransistor PTR₃₁ are utilized to control the switching period of the switching transistor TR₃₁ by detecting the current. The resistor R₄₀, zener diode ZD₃₁ and diode D₃₅ are utilized to control the primary side input voltage variation. The resistors R₃₉ and R₄₂, capacitor C₃₂ and diode D₃₄ are utilized to protect the power supply source and secondary load from an overcurrent state.

The operation of the above embodiment according to the present invention will be explained in detail below.

When an over current is present such that a load which is supplied with second constant voltage V₃₂ is short circuited or the like, a voltage drop at the resistor R₄₅ becomes larger and a lower potential is applied to a base of the transistor TR₃₃. Therefore, the transistor TR₃₃ becomes conductive, and accordingly, the photodiode PD₃₂ becomes lit. Since the phototransistor PTR₃₂ which senses the light of the photodiode PD₃₂ the power of the power input terminal V_(IN) is applied to a gate of the thyristor SCR₃₁ through the resistor R₃₂ and phototransistor PTR₃₂. The thyristor SCR₃₁ is then turned ON. The power applied to a base of the switching transistor TR₃₁ flows through the diode D₃₂ and thyristor SCR₃₁ ; therefore the switching operator of the transistor TR₃₁ is not executed.

As explained in detail above the present invention presents the power from being unnecessarily lost because the switching operation of the switching transistor is interrupted by detecting when an overvoltage is outputted. 

What is claimed is:
 1. A protection circuit for a switching mode power supply circuit which a voltage from a voltage input terminal is induced in first and second secondary coils of a transformer and a voltage is induced in a third secondary coil of the transformer for controlling ON/OFF switching of a switching transistor through a feedback circuit, the voltage induced in the first and second secondary coils being rectified by first and second rectifiers and outputted with first and second constant voltages, comprising:a photodiode being connected to the second rectifier through a zener diode, a first thyristor having an anode connected to the first rectifier; a phototransistor which senses light produced by said photodiode; a capacitor being connected to said phototransistor; and a second thyristor being connected to said phototransistor; being connected to the power input terminal; said thyristor being connected to the power input terminal; said switching transistor being connected to the power input terminal through a diode; wherein said ON.OFF switching operation of said switching transistor is interrupted when an overvoltage is outputted from said second rectifier, said overvoltage being a voltage higher than a zener voltage of said zener diode.
 2. A protection circuit for a switching mode power supply circuit which a voltage from a voltage input terminal is induced in first and second secondary coils of a transformer and a voltage is induced in a third secondary coil of the transformer for controlling ON/OFF switching of a switching transistor through a feedback circuit, the induced voltage in the first and second secondary coils being rectified by first and second rectifiers and outputted with first and second constant voltages, comprising:a first photodiode being connected to the second rectifier through a first transistor; a first zener diode being connected in parallel with said first photodiode and said first transistor through a first resistor; a second transistor being connected to the first rectifier through a second photodiode; a first phototransistor which senses light produced by said first photodiode; a capacitor being connected to said first phototransistor; a thyristor being connected to said first phototransistor; a second phototransistor being which senses light produces by said second photodiode; and a second zener diode being connected in parallel with said second phototransistor; said first and second phototransistors being connected to said switching transistor through a first and second diode, respectively; said first phototransistor and said thyristor being connected to the power input terminal; said ON/OFF switching operation of said switching transistor being interrupted when an overcurrent is detected by said first transistor and said first zener diode; said overcurrent causing said first transistor to turn on, thereby causing said first photodiode to illuminate. 